1. Field of the Invention
The present invention generally relates to ionic polymer transducers, sensors and actuators.
2. Description of the Prior Art
Ionic polymers membranes are materials that behave as solid-state electrolytes, making them useful in a variety of applications, including fuel cells, water electrolyzers, transducers, actuators and sensors. Ionic polymer membrane actuators have existed in their current state since the early 1990s. See, for example, Sadeghipour, K., Salomon, R., and Neogi, S., 1992. “Development of a Novel Electrochemically Active Membrane and ‘Smart’ Material Based Vibration Sensor/Damper.” Smart Materials and Structures, 1, pp. 172-179; Oguro, K., Kawami, Y., and Takenaka, H., 1992. “Bending of an Ion-Conducting Polymer Film-Electrode Composite by an Electric Stimulus at Low Volage.” Journal of Micromachine Society, 5, pp. 27-30.; and U.S. Pat. Nos. 5,268,082 to Oguro, 6,109,852 to Shahinpoor, and 6,475,639 to Shahinpoor, each of which are herein incorporated by reference. These devices are often based on a perfluorosulfonic or percarboxylic acid membrane backbone with charged side groups. The addition of these charged groups allows the polymer membrane to conduct counterions and thus serve as a polymer electrolyte. This property also allows the polymer membrane to behave as a distributed actuator or sensor due to the redistribution of the cations within the polymer under the application of an electric field or when a stress is applied to the membrane.
In order for this phenomenon to occur, two conditions must be met. First, the cations must be free to move about within the polymer matrix. This is done by saturating the polymer with a solvent. The solvent of choice prior to this invention was water; however, U.S. Pat. No. 5,268,082 discusses the use of salt water and physiologic salt water and indicates that displacement with salt water is smaller than when operated in pure water. The solvent dissolves the cations associated with the pendant acidic groups and allows them to move within the polymer. Second, the surfaces of the membrane must be made to conduct electricity. This is accomplished by depositing a thin metal electrode on both surfaces of the membrane.
Typically, the ion exchange properties of the polymer are used to facilitate the deposition of the metal. The polymer is pretreated by sandblasting, hydrating, and cleaning in a strong acid. This acid wash also serves to ensure that the polymer is fully saturated with protons. The membrane is then placed into an aqueous solution containing ions of the metal to be plated. These ions are allowed to exchange with the protons in the polymer for a predetermined amount of time and are then reduced to their neutral state at the surface of the polymer by a reducing agent (typically NaBH4 or LiBH4). In this solvated and electroded form, the ionic polymer membrane can be made to bend towards the anode side when a small electric field (1-5 V) is applied across its thickness, thus making it a soft, distributed actuator. These membranes in this form can also be used as distributed sensors since the transient voltage generated across the membrane is correlated to the quasi-static displacement of the membrane. More recently, Newbury and Leo have shown that the charge generated at the surfaces of the membrane is proportional to the strain in the material (see, Newbury, K. and Leo, D. J., 2002, “Electromechanical Modeling and Characterization of Ionic Polymer Benders.” Journal of Intelligent Material Systems and Structures, 13(1), pp. 51-60).
Ionic polymer membrane actuators have several advantages over other types of actuation technology. For example, because they are soft and saturated with water, they are amenable to implantation in the human body and therefore have significant potential in biomedical applications. Also, as compared to other many types of smart materials, these actuators generate larger strains (>1%) and strain rates at lower applied potentials (<10 kV/m) than over types of electroactive devices based on the piezoelectric or electrostrictive effect.
However, there are some key limitations to currently available ionic polymer membrane technology that has prevented it from experiencing widespread use. One of the most problematic limitations is the dependence of ionic polymer transducers on a solvent in order to function. As noted above, pure water (and sometimes salt water or physiologic water) is used as the solvent, but because the water will quickly evaporate in air, the applications for these devices are limited by its use. Also, the water will decompose into hydrogen and oxygen gas once the electrolysis voltage limit is reached (around 1 V). This decomposition will also contribute to rapid loss of the water and a corresponding drop in the performance of the polymer transducer.
One way around this problem would be to use a barrier coating to contain the water inside the membrane. Bar-Cohen et al reported that with the aid of a barrier coating they were able to operate a sample in air for four months (see, Bar-Cohen, Y., Leary, S., Shahinpoor, M., Harrison, J., and Smith, J., 1999, “Flexible Low-Mass Devices and Mechanisms Actuated by Electroactive Polymers” in EAP Actuators and Devices. SPIE, vol. 3669, pp. 51-56, and Bar-Cohen, Y., Leary, S., Shahinpoor, M., Harrison, J. O., and Smith, J., 1999. “Electro-Active Polymer (EAP) Actuators for Planetary Applications” in EAP Actuators and Devices. SPIE, vol.3669, pp.57-63). However, such a barrier coating will add passive stiffness to the actuator device and reduce the amount of strain that the device can generate. Furthermore, if the electrolysis limit of the aqueous solvent is exceeded, the formation of hydrogen and oxygen gases at the membrane surfaces will create blisters beneath the barrier coating and lead to de-lamination of this coating from the polymer transducer.
Although most of the work in the area of ionic polymer transducers has focused on water as the solvent, there have been a limited number of studies on the use of alternative solvents. For example, Nemat-Nasser has demonstrated the use of ethylene glycol and glycerol as suitable solvents for these materials (see, Nemat-Nasser, S., 2002. “Micromechanics of Actuation of Ionic Polymer-metal Composites” Journal of Applied Physics, 92(5), pp. 2899-2915, and Nemat-Nasser, S., 2003. “Experimental Study of Nafion-and Flemion-based Ionic Polymer-Metal Composites (IPMCs) with Ethylene Glycol as Solvent” in EAP Actuators and Devices. SPIE). These materials do not suffer from the dehydration problem associated with water, but the speed of the actuation mechanism in the transducers is reduced significantly. Also, Shahinpoor and Kim have demonstrated that composites of poly(ethylene oxide) and poly(ethylene glycol) will exhibit electromechanical behavior under an applied electric field with no aqueous solvent necessary (see, Shahinpoor, M. and Kim, K., 2001. “Fully Dry Solid-State Artificial Muscles Exhibiting Giant Electromechanical Effets” in EAP Actuators and Devices. SPIE, pp. 428-435, and Shahinpoor, M. and Kim, K., 2002. “Solid-State Soft Actuator Exhibiting Large Electromechanical Effect” Applied Physics Letters, 80(18), pp. 3445-3447). In these materials, the very low molecular weight poly(ethylene glycol) essentially serves as the solvent, creating soft amorphous phases in the composite polymer that facilitate motion of the cations.